In "I Felt a Funeral in my Brain," Emily Dickinson's controlling metaphor of a funeral is sufficiently ambiguous that it can be interpreted in more than one way. Thus, two themes, or common points, emerge: Madness and Insanity and Doubt and Uncertainty.
Madness and Insanity
Interpreting the funeral as the end of rationality, the reader perceives the final stanza with its metaphoric "Plank of Reason" breaking as a mental breakdown, with the patient plunging into an unknown state of mind that is irrational:
And I dropped down, and down--
And hit a World, at every plunge,
The "funeral," then, is the end of the speaker's life as a rational being. For, she is now "buried" and "solitary" in her own disturbed "World." And, her mental anguish is conveyed interestingly with the "Boots of Lead" and beating "like a Drum" in her head.
Doubt and Uncertainty
In another interpretation of the metaphoric "funeral," the speaker may refer to the death of some feeling, or belief such as religious faith. With this interpretation, the religious ceremony of a funeral is an ironic metaphor that compares the death of faith with the religious death rite. Here, then, the "Mourners to and fro" are the various tenets of the speaker's religious beliefs that are being trod upon with the "Bell" as the final and inescapable loss of faith, the point in which the mind rejects beliefs. The rational recognition of this loss, then, is compared to the "Finished knowing" as the "Plank in Reason" breaks.